Continuous observation of rapidly growing, healthy colonies of Campanularia flexuosa shows that hydranths are short lived: after a few days a hydranth loses its form; its substance passes into the coenosarc of the colony; its hydrotheca drops off; and, in a day or two, a new hydranth, with a short pedicel, develops at the tip of the old pedicel. Hydranths live for about 4 days at 21[degree]C, 7 at 17[degree] C. The process is orderly: older hydranths regress before younger ones. A 5-day starvation period accelerated the regression cycle but did not alter the orderliness of the sequence. Prepared slides of Obelia show stages of regression and replacement. They also show orderliness of the regression in that older, more basal pedicels have perisarcal annulations of several distinct thicknesses[long dash]the record left by several cycles of regression and replacement. Other calyptoblasts show evidence of a naturally occurring regression-replacement cycle; gymnoblasts do not. The process is significant in that it insures a youthfulness of the feeding members of a colony, rids the colony of organisms attached to the hydrothecae, and increases the pedicel length of more basal hydranths.